docs: refactor build section

Under "Getting Started", created dedicated subsections for:
- Prerequisites
- Get Source
- Building

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I46f7b924f812aea6f3bd3a9216fe2ccbec0287e8
diff --git a/docs/getting_started/index.rst b/docs/getting_started/index.rst
index c7ca45e..dc780a2 100644
--- a/docs/getting_started/index.rst
+++ b/docs/getting_started/index.rst
@@ -5,8 +5,9 @@
    :maxdepth: 1
    :caption: Contents
 
+   prerequisites
+   get-source
    building
-   docs-build
    coding-style
    commit-style